Protein kinase activity of RNA polymerase I purified from a rat hepatoma: probable function of Mr 42,000 and 24,600 polypeptides

Abstract

RNA polymerase I was purified to homogeneity from Morris hepatoma 3924A. Purified RNA polymerase I contained a protein kinase activity but comigrated with the polymerase in nondenaturing gels. RNA polymerase II, purified from the same hepatoma, lacked protein kinase activity. Analysis of the subunit composition of the RNA polymerase I showed the presence of eight polypeptides: S1, Mr 190,000; S2, Mr 120,000; S3, Mr 62,000; S4, Mr 42,000; S5, Mr 24,600; S6, Mr 21,000; S7, Mr 19,500; and S8, Mr 17,500. Antibodies prepared against purified polymerase I specifically inhibited RNA synthesis catalyzed by RNA polymerase I. When subunits of the enzyme were covalently linked to diazobenzyloxymethyl paper, complexes between the antibody preparation and S1-S6 were visualized. No immune complexes were formed between RNA polymerase I antibodies and RNA polymerase II subunits. The antibody preparation was able to inhibit both the protein phosphorylation catalyzed by RNA polymerase I and that catalyzed by a nuclear kinase (NII) purified from the same hepatoma. The two polypeptides of the nuclear kinase--Mr 42,000 and 24,600 (identical in size to S4 and S5 of polymerase I)--formed visible complexes with the RNA polymerase I antibodies. Both S4 and S5 of the polymerase contained an ATP binding site, a property associated with protein phosphorylation and also exhibited by the polypeptides of the purified kinase. These data suggest that polypeptides of Mr 42,000 and 24,600 associated with polymerase I are responsible for its kinase activity.